1. Field of the Invention
The present invention generally relates to a high frequency amplifier circuit for high frequency amplifying a signal received through an antenna, and particularly relates to a high frequency amplifier circuit, for example, provided in an antenna input circuit for an FM receiving apparatus, for example, as an antenna booster circuit in order to improve the receiving sensitivity of the FM receiving apparatus.
2. Background Art
A conventional high frequency amplifier circuit of the type described above is in the form of a negative feedback amplifier circuit A with its input directly connected to an antenna AMI, as shown in FIG. 1. The negative feedback amplifier circuit has advantages in that a wide band frequency characteristic suitable as an antenna input circuit can be obtained owing to its negative feedback, noise immunity can be reduced, and so on. In addition to these advantages, the negative feedback amplifier circuit has the feature that by making its negative feedback rate large (i.e., equal to or near 1 (one)) it is possible to make its input impedance high and its output impedance low to thereby make the internal resistance of the negative feedback amplifier circuit viewed from the output side very small so that the circuit is hardly subject to the influence of load impedance.
On the other hand, it is required to increase the gain of a high frequency amplifier circuit without increasing its noise figure. To satisfy this requirement, it is necessary to make the negative feedback amplifier circuit A have a low noise and high gain characteristic. If an amplifier circuit is made to have such a characteristic however, the distortion generated on reception of an intensive input (at the time of overload) becomes large. In order to minimize the distortion generated on reception of an intensive input, it is necessary to make the feedback rate of the negative feedback amplifier circuit A very large. However, it is impossible to obtain a sufficient gain.
In short, in the conventional circuit described above with reference to FIG. 1, there has been the problem in that if its negative feedback rate is made large, the noise figure NF thereof expressed by the respective ratios of Si/Ni and So/No at the input and output sides of the amplifier circuit, respectively, is so deteriorated that its practical sensitivity does not rise sufficiently to a value required for such a circuit.